This invention relates generally to image carrying transparencies and a method for making same and more particularly, concerns the provision of transparencies formed electrophotographically and comprising a transparent plastic sheet substrate, a thin ohmic layer bonded thereto and a thin overcoat layer of a compatible resinous material itself carrying a toner image embedded within said resinous material. The transfer is effected electrophoretically without residual material remaining on the electrophotographic recording member, the overcoating being softened subsequent to receipt of the toned image to effect said embeddment.
In U.S. Pat. No. 4,025,339 an electrophotographic member has been described which includes a photoconductive coating formed of a crystalline deposit of a wholly inorganic material on a suitable substrate, such as a thin clear flexible plastic sheet with an intervening thin ohmic layer such as tin-indium oxide, in a sputtering method that results in unusual properties. The photoconductive coating has the ability to function as means for storing and selectively discharging an electrical charge. When exposed to light the radiation pattern forms a latent electrostatic image in the photoconductor surface which is toned to render same visible.
It is known to transfer the toned electrostatic images to a receiving member which may comprise a thin transparent plastic sheeting. Transfer can be effected employing dry transfer methods conventionally described as cascade toning or development employing dry electroscopic toner particles. Alternatively, development of the electrostatic latent image can be effected using liquid toning techniques wherein toner particles are dispersed in suspension in an insulating hydrocarbon liquid. Liquid toner techniques offer improved resolution due to the small size of the dispersed particles. Accordingly, in order to take advantage of the unusual properties of the patented electrophotographic recording member, liquid toning methods conventionally are used.
Transfer of toned images directly to sheet plastic receiving members has been successful. Bias voltages have been employed during the transfer with success. Notwithstanding the production of transparency sheets using those known methods, adherence of the toner images on the substrate surface has been less than desirable. The advent of fusible toner particles alleviated the adhesion problem to some degree but the result was a surface which was not sufficiently smooth for use of the resultant transparency, say for optical projection. Further, the high resolution obtained using the patented electrophotographic recording member, as well as the optical density of the resultant image, was not always fully retained upon transfer. Lateral displacement was a problem.
Incomplete transfer often resulted, leaving a residue toner on the recording member, necessitating additional steps of carefully cleaning the electrophotographic surface of such residual deposit before reuse. The recording member itself could be used as a transparency providing the substrate was transparent but the photoconductive coating usually carried thereby has a characteristic tint to the substrate, yellow in color where CdS is used as the coating material. Such transparencies as result by fusing a toned transferred image onto the electrophotographic member are unsatisfactory because of such tint and because they are expensive in view of the cost of the recording medium.
Adhesive substances have been applied as a coating to sheet material but do not result in transfer without loss in resolution and optical density at selected areas. The transfers obtained are not satisfactory because the adhesive tends to result in so-called blocking, that is, bonding of the sheets together when placed thereupon even when cool. The blocking effect required the lamination of an additional thin foil of plastic over the image to protect it against blocking, dust collection and scratching. Unfortunately, the thin overlay foil eliminates the making of contact duplicate films, a vital necessity in the film business, as it prevents direct image contact because the foil acts as a spacer. The resulting resolution loss introduced by light scatter in the space located by the foil becomes unacceptable.